1. Field of the Invention
The present invention relates generally to improvements and telephone modems and, more particularly, pertains to new and improved digital telephone modems with line protection devices.
2. Description of Related Art
A telephone modem can be seriously damaged if it is accidentally connected to a digital Private Branch Exchange (PBX) terminal which are readily found in office environments. A digital PBX terminal (DPBX) looks and acts just like a standard RJ11 jack that is used for the public switch telephone network (PSTN), or an analog PBX. A PSTN terminal is equivalent to a 20 to 100 volt battery at the central office in series with a loop resistance which can vary between 200 ohms and 5,000 ohms. A DPBX terminal, on the other hand, is equivalent to a 10 to 100 volt battery with a resistance of less than 10 ohms. If a telephone modem is connected to the digital PBX terminal, the current through the modem will exceed the maximum expected limits without any drop in voltage at the tip and ring terminals. The resulting excess power will permanently damage the line interface circuit of the modem. Attempts to prevent damage to a telephone modem which is accidentally connected to a DPBX jack have used such devices as optoisolators to detect excessive current flow through the line interface circuitry of the modem. When excessive current is detected in this manner, the modem goes xe2x80x9con-hook,xe2x80x9d thereby protecting the circuit from being damaged. Although these prior art devices may be satisfactory in their operation, the additional hardware required to implement the line protection function increases the cost of the modem inordinately. Furthermore, these prior art solutions cannot be used in a line interface configuration which limits the line current in itself. The present invention, on the other hand, uses the software-based architecture of its modem to implement a line protection device without adding additional hardware and is well-suited for use with all interface circuits.
To prevent damage to a modem that is accidentally connected to a digital PBX terminal which looks just like a standard RJ11 jack of the PSTN, the modem is equipped with software routines that use voltage readings from the modem telephone line interface circuit to identify a PBX connection and cause the modem to go on-hook. The ADC (analog-to-digital converter) of the modem measures the t-r line voltage through a resistor divider network. Based on this voltage, the software calculates either the power through the interface in off-hook state, or the rate of change of the line voltage over time while going off-hook, or the loop resistance of the PSTN. If the power is too high for the interface, the voltage rate of change is less than a predetermined amount or the loop resistance is less than an expected minimum, the software directs the modem to go on-hook.